From prior practice drivetrains of vehicles, in particular off-road vehicles, are known, which comprise a drive engine, a multi-range transmission and a drive output. The incorporation of a multi-range transmission enables several gear speeds to be made available with the fewest possible gear wheel pairings.
In practice, a multi-range transmission consists of a combination of several transmission units, such as a so-termed upstream reduction gear group, a main transmission and a downstream group or range transfer box. To provide good shifting comfort and as many speed shifts as possible, the main transmission is preferably made as an automatic transmission, for example with six forward speeds and one reverse. The combination of such an automatic transmission with a range transfer box downstream therefrom extends the gear-speed sequence of a multi-range transmission consisting of a main transmission and a range transfer box.
As a transmission unit, a range transfer box is characterized in that an input rotation speed of the range transfer box is always converted to “slow”. Moreover, it is characteristic of a range transfer box that in the range transfer box a torque increase takes place so that vehicles or off-road vehicles can still be driven even on steep slopes. Owing to the large torque increase, range transfer boxes are always connected downstream from a main transmission, which avoids the transfer of high torques through the main transmission itself.
In practice, range transfer boxes are made as reduction gear systems or compact planetary gear sets. The transmission ratio in a range transfer box is changed by way of shift elements with which the ratio is shifted between a first range (“low”) and a second range (“high”). When the first (“low”) range is engaged in the range transfer box in combination with the main transmission, this provides the driver with a transmission ratio range that is advantageous for the operation of a vehicle on land with steep slopes at low vehicle speeds.
The second (“high”) transmission range of the range transfer box is less affected by losses, so that when driving over normal territory and also at higher driving speeds the second (“high”) range is preferably engaged in the range transfer box.
When shifting between the two transmission ranges “low” and “high” of the range transfer box, synchronization of the shift elements of the range transfer box to be engaged for the respective “low” or “high” range desired is necessary, since owing to the large ratio change in the range transfer box between shift elements to be disengaged and ones to be engaged, large differential rotation speeds exist. These speed differences are equalized by suitable synchronization means, which in the range transfer box can either be made as separate components or integrated in the shift elements of the range transfer box. In the latter case, the shift elements are usually made as frictional disk clutches or disk brakes.
This has the disadvantage, however, that range transfer boxes constructed with separate mechanical synchronization means or with frictional shift elements comprise large components and take up a lot of structural space, since they have to be made appropriately robust because of the high stressing of the components during synchronization. Furthermore, separate mechanical synchronization means or frictional shift elements incur high manufacturing costs, and this is uneconomical.
Accordingly, the purpose of the present invention is to provide a method for controlling the drive train of a vehicle with which the transmission ratio of a range transfer box can be changed by way of a more compact and less costly structure.